Motor-driven traverse rods have been in use for many years. Initially, these motor-driven traverse rods were expensive and difficult to install. Recent innovations have now made these devices affordable and suitable for consumer installation.
These new traverse rod assemblies comprise adjustable telescoping rods upon which are mounted first and second master carriers and numerous idler carriers. The master carriers are driven by a flexible, perforated, relatively non-stretchable drive tape which extends from end-to-end within the rod in the form of a loop. Pulleys are provided at each end of the rod and the tape loop passes around them with the ends of the loop joined at the first master carrier. At one end of the rod the tape also passes around a toothed sprocket, with the teeth of the sprocket fitting, in positive driving relation, into the perforations in the tape. The sprocket is driven from the outside by a motor or other drive mechanism to control the position of the master carriers. Adjustable means, accessible from the outside of the rod, are provided for securing the ends of the tape loop to the first carrier so that the length of the rod can be adjusted, and, thereafter, all substantial slack can be removed from the tape loop. Within the rod, the other leg of the tape loop is secured to the second master carrier at the point on the tape loop which corresponds to the fully opened position of the second master carrier when the first master carrier is in the fully opened position.
By the nature of the system, fine adjustments of the drive tape are not easily made. As a result, it is not always possible to have the drive tape fit snugly over the toothed sprocket.
Also, the drive tape bends more easily at the areas where there are perforations than at the areas between the perforations. Thus, when the tape is bent, as when it passes around a circular drive sprocket, the tape unless it is heavily tensioned, does not form a smooth curved surface, but rather appears more like a series of short but relatively straight sections hinged together. A profile of the tape as it passes over a conventional sprocket is shown in FIG. 3 and illustrates this effect.
Because of the non-circular bending characteristic of the drive tape, when the tape-sprocket assembly is not under high tension the tape will not lie flush against the portions of a conventional sprocket between the sprocket teeth and an unpleasant rattling noise in operation results. Not only is the noise unpleasant but undersired wear on the tape and sprocket result.
A basic objective of this invention is to provide an inexpensive means for allowing the perforated tape to fit smoothly over the toothed sprocket, so as to eliminate the rattle and wear just described. A more specific objective is to provide a toothed sprocket that is adapted to better fit the drive tape as the tape passes over the sprocket.